Freelance professional copy editor
I work mostly on postgraduate textbooks in the humanities and social sciences, but I also work directly with new authors of fact and fiction.
In no particular order, the subjects I copy edit include: politics, international relations, economics, economic geography, transport geography, geomorphology, education, media studies, business, research methods, ecology, linguistics and psychology. I’ll turn my hand to most things, but know when I won’t add value so I don't take on jobs in the hard sciences, for instance.
The English language is a slippery thing, constantly evolving.† It's important your copy editor has an ear for that change and growth, so that your writing is right but also current and appropriate for the intended readership.
In short, a professional copy editor can help you make sure the written word doesn't let you down.
†Here's an example of just how slippery English can be, using a term close to my heart. Copy editor. Copyeditor? Copy-editor? Copyedit, copy edit, copy-edit? Well, yes - they can all be right!
The Oxford English and Collins dictionaries choose copy-edit but copy editor. Chambers doesn't have it at all, which means copy edit, copy editor. The Society for Editors and Proofreaders hyphenates both - copy-edit, copy-editor. Many online dictionaries have the combined form - copyedit, copyeditor - and this form is most common in US and Australian dictionaries. The Canada Space dictionary has copy-editor but copyedit.
Many other words have the same jungle to fight through, and copy editors/copy-editors/copyeditors are here to help.